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The smallest amount of a specific chemical substance. Large molecules such as proteins, fats, carbohydrates, and nucleic acids are the building blocks of a cell, and a gene determines how each molecule is produced.

FK Reading Ease
46.5

FK Grade Level
High School
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    A subset of T cells and B cells that have been exposed to antigens and can then respond more readily when the immune system encounters those same antigens again.

    FK Reading Ease
    49

    FK Grade Level
    College
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    A granulocyte found in tissue. The contents of mast cells, along with those of basophils, are responsible for the symptoms of allergy.

    FK Reading Ease
    57.2

    FK Grade Level
    Middle School
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    A group of genes that controls several aspects of the immune response. MHC genes code for “self” markers on all body cells.

    FK Reading Ease
    76.5

    FK Grade Level
    Elementary School
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    A large and versatile immune cell that devours invading pathogens and other intruders. Macrophages stimulate other immune cells by presenting them with small pieces of the invaders.

    FK Reading Ease
    33.3

    FK Grade Level
    High School
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    Powerful chemical substances secreted by lymphocytes. These molecules help direct and regulate the immune responses.

    FK Reading Ease
    13.1

    FK Grade Level
    College
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    An organ of the immune system where lymphocytes develop and congregate. These organs include the bone marrow, thymus, lymph nodes, spleen, and various other clusters of lymphoid tissue. Blood vessels and lymphatic vessels are also lymphoid organs.

    FK Reading Ease
    50.3

    FK Grade Level
    High School
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    A small white blood cell produced in the lymphoid organs and essential to immune defenses. B cells, T cells, and NK T cells are lymphocytes.

    FK Reading Ease
    75.7

    FK Grade Level
    Elementary School
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    A bodywide network of channels, similar to the blood vessels, which transports lymph to the immune organs and into the bloodstream.

    FK Reading Ease
    52.6

    FK Grade Level
    High School
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    A small bean­shaped organ of the immune system, distributed widely throughout the body and linked by lymphatic vessels. Lymph nodes are garrisons of B and T cells, dendritic cells, macrophages, and other kinds of immune cells.

    FK Reading Ease
    52.3

    FK Grade Level
    High School
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